Method and system for fuel vapor control

Abstract

Methods and systems are provided for identifying leaks on each of a fuel tank and a canister side of a fuel system using a single leak check module. A position of a switching valve coupled in the vapor line of the fuel system is adjusted to selectively couple the leak check module to either the fuel tank or the canister. Leak is detected based on a change in pressure at a reference orifice of the leak check module following applying of vacuum from a vacuum pump.

The invention claimed is: 
     
       1. A method, comprising:
 indicating leakage on a canister side of a fuel system based on a first change in pressure at a reference orifice following applying of vacuum to the fuel system by operating a vacuum pump with an isolation valve closed; and 
 indicating leakage on a fuel tank side of the fuel system based on a second change in pressure at the reference orifice following applying of vacuum to the fuel system with the isolation valve open, an inlet of the pump coupled to the fuel system via a three-way switching valve, where in a first valve position the pump inlet coupled directly to the canister while the isolation valve is closed, and in a second valve position the pump inlet is coupled directly to the tank with the isolation valve open, the change in pressure detected by at least a fuel system pressure sensor. 
 
     
     
       2. The method of  claim 1 , wherein the isolation valve is coupled between a fuel tank and a canister of the fuel system. 
     
     
       3. The method of  claim 2 , wherein indicating leakage on the canister side based on the first change in pressure includes indicating no leakage on the fuel tank side. 
     
     
       4. The method of  claim 2 , wherein indicating leakage on the fuel tank side based on the second change in pressure includes indicating no leakage on the canister side. 
     
     
       5. The method of  claim 2 , wherein indicating leakage based on the first change in pressure includes indicating leakage based on the first change in pressure being larger than a first threshold, the first threshold based on a first threshold leak size on the canister side relative to the reference orifice. 
     
     
       6. The method of  claim 5 , wherein indicating leakage based on the second change in pressure includes indicating leakage based on the second change in pressure being larger than a second threshold, the second threshold based on a second threshold leak size on the fuel tank side relative to the reference orifice. 
     
     
       7. The method of  claim 6 , wherein the second threshold leak size on the fuel tank side is smaller than the first threshold leak size on the canister side, and wherein the second threshold is higher than the first threshold. 
     
     
       8. The method of  claim 6 , wherein the vacuum pump and the reference orifice are included in a leak check module of the fuel system, the leak check module coupled to the fuel system via the switching valve, the switching valve coupled between the canister and atmosphere and the fuel tank and atmosphere. 
     
     
       9. The method of  claim 8 , wherein indicating leakage based on the first change includes actuating the switching valve to a first position and wherein indicating leakage based on the second change includes actuating the switching valve to a second, different position. 
     
     
       10. The method of  claim 9 , wherein when the switching valve is actuated to the first position, vacuum is applied from the vacuum pump directly to the fuel tank, and wherein when the switching valve is actuated to the second position, vacuum is applied from the vacuum pump directly to the canister. 
     
     
       11. A method for monitoring and operating a fuel system coupled to an engine, comprising:
 operating in a first mode with a switching valve in a first position to detect leakage on a first, canister side of the fuel system with vacuum applied from a vacuum pump directly to the fuel tank; and 
 operating in a second mode with the switching valve in a second position to detect leakage on a second, fuel tank side of the fuel system, with vacuum applied from the vacuum pump directly to the canister, the fuel tank coupled to the canister on the tank side, the canister communicating with atmosphere on the first side. 
 
     
     
       12. The method of  claim 11 , further comprising distinguishing fuel system degradation due to leakage on the first, canister side from fuel system degradation due to leakage on the second, fuel tank side of the fuel system based on a first change in fuel system pressure while operating in the first mode relative to a second change in fuel system pressure while operating in the second mode. 
     
     
       13. The method of  claim 12 , wherein the fuel system includes a fuel tank coupled to a canister via an isolation valve, and wherein the switching valve is a three-way valve located between the fuel tank and the isolation valve, the switching valve coupling each of the fuel tank and the canister directly to a leak check module including the vacuum pump and a reference orifice, the three-way valve coupled directly to the fuel tank in a first passage between the isolation valve and the tank in a first position, and coupled directly to the canister in a second passage separate and upstream of the first passage, the second passage connected coupling the three-way valve to the canister. 
     
     
       14. The method of  claim 13 , wherein operating in the first mode further includes operating with the isolation valve closed and wherein operating in the second mode further includes operating with the isolation valve open. 
     
     
       15. The method of  claim 14 , wherein operating in the first mode further includes selectively applying vacuum from the vacuum pump to only the first, canister side of the fuel system and indicating leakage on the first side based on the first change in pressure at the reference orifice being higher than a first threshold, and wherein operating in the second mode further includes selectively applying vacuum from the vacuum pump to only the second, fuel tank side of the fuel system and indicating leakage on the second side based on the second change in pressure at the reference orifice being higher than a second threshold. 
     
     
       16. The method of  claim 15 , wherein the first threshold is based on a first threshold leak size to be detected on the canister side and the second threshold is based on a second threshold leak size to be detected on the fuel tank side, and wherein the first threshold is lower than the second threshold. 
     
     
       17. A fuel system, comprising:
 an engine; 
 a fuel tank; 
 a canister for storing fuel vapors; 
 an isolation valve coupled in a vapor line between the fuel tank and the canister; 
 a leak check module including a reference orifice, a vacuum pump, and a pressure sensor; 
 a three-way switching valve coupled to each of a vent of the canister and a separate conduit from the canister, the switching valve selectively couplable directly to the fuel tank and the canister via the conduit and vent, respectively; and 
 a controller with computer readable instructions for:
 in response to leak detection conditions being met,
 closing the isolation valve; 
 actuating the switching valve to a first position to selectively couple the leak check module to the canister; 
 operating the vacuum pump to apply vacuum to the canister; and 
 indicating a canister leak but not a fuel tank leak based on a first change in pressure at the reference orifice following the applying of vacuum being larger than a first threshold. 
 
 
 
     
     
       18. The fuel system of  claim 17 , wherein the controller includes further instructions for:
 opening the isolation valve; 
 shifting the switching valve to a second position to selectively couple the leak check module to the fuel tank; 
 operating the vacuum pump to apply vacuum to the fuel tank; and 
 indicating a fuel tank leak but not a canister leak based on a second change in pressure at the reference orifice following the applying of vacuum being larger than a second threshold. 
 
     
     
       19. The fuel system of  claim 18 , wherein the controller includes further instructions for:
 setting a first diagnostic code in response to the indication of canister leak; and 
 setting a second diagnostic code in response to the indication of fuel tank leak.